LAGOS, Nigeria — An international watchdog said Tuesday it will cost about $4 million to clean up toxic lead and secure mines in northern Nigeria, where activists say "the worst outbreak of lead poisoning in modern history" has taken place.

At least 400 children have died since March 2010, and thousands more continue to be exposed to dangerously high levels of lead, said Human Rights Watch researcher Jane Cohen.

The children are being exposed while processing ore in these informal mines not owned by any company, or when their miner relatives return home covered with lead dust. The children's food and surroundings also have been contaminated when people crush and grind rocks at home to extract the ore.

Amina Murtala, 20, who lives in Bagega, one of the largest and worst affected villages in Zamfara state, told Human Rights Watch that three of her six children had died from lead poisoning. Tests in her compound showed it has 60 times more lead than safe levels.

"Each time one died, I was so distraught and so traumatized," Murtala says in a Human Rights Watch video in the local Hausa language.

Nearly two years after the problem began, women are still sorting groundnuts on the ground, exposing their food to contamination.

"People are more aware (of the causes of the child deaths), but there is a disconnect and a sense that there are no alternatives," said Cohen, who just returned from the hardhit area.

Extreme poverty in this corner of Nigeria's arid northwest, where people live in mud houses and depend on a likely contaminated pond for water, means that communities have continued to mine in spite of the threat of poisoning.

Children under the age of 5 are particularly at risk of lead poisoning, which can damage their nervous and muscular systems. Human Rights Watch says there also have been high rates of infertility and miscarriages among affected women.

Last May, Doctors Without Borders said it collaborated with the Nigerian government, the U.S. Centers for Disease Control and Prevention, the World Health Organization, UNICEF and TerraGraphics/Blacksmith Foundation to decontaminate seven of the affected villages and treat 1,500 children.

However, the group said there were limits to what it could do as an emergency aid organization, after the government had focused mainly on raising awareness.

Now, Human Rights Watch is asking the government of this oil-rich West African nation to commit funds to make the gold mining processes safer, clean up all the affected villages, and test and treat all at-risk children.

The group wants the funds to come quickly so that work can be done by June, ahead of the raining season expected to worsen contamination.

"There have been many visits by top officials, but no real commitment seen," said Human Rights Watch Deputy Program Director Babatunde Olugboji.